Field of the invention
The present invention relates to an engine intake system and more particularly to an air-fuel ratio control for an engine wherein the air-fuel ratio is controlled to a desirable value in accordance with outputs of a sensor which detects components of exhaust gas.
Description of Prior Art
There is known an intake system for an internal combustion engine in which an O.sub.2 sensor detects a change of oxygen concentration in exhaust gas so that the air-fuel ratio is controlled based on the output of the sensor.
For example, Japanese Patent Public Disclosure No. 59-208141 filed on May 12, 1983 and diclosed for public inspection on Nov. 26, 1984, discloses a method of controlling lean air-fuel ratio in electronic control engine in which the air-fuel ratio is controlled to the leaner side than the theoretical air-fuel ratio in response to the output of a lean sensor for generating a signal proportional to oxygen concentration in exhaust gas by means of a feedback control. In Japanese Public Disclosure No. 57-203844 filed on June 10, 1981 and diclosed on Dec. 14, 1982 for public inspection, there is disclosed an engine air-fuel ratio control system wherein an air-fuel feedback control system is effected to obtain a desirable air-fuel ratio by employing a linear O.sub.2 sensor which produces outputs proportional to the oxygen concentration in the exhaust gas. Further in the Japanese Patent Public Disclosure No. 58-27847 filed on Aug. 13, 1981 and disclosed on Feb. 18, 1983 for public inspection, there is disclosed a feedback control system for the air-fuel ratio control in which the feedback control is carried out even in the warming-up condition of the engine.
In the system disclosed in the Japanese Patent Public Disclosure No. 59-208141, there is provided a map by which a base injection pulse width is obtained based on the engine speed and intake gas pressure in the intake passage. A desirable air-fuel ratio corresponding to an engine operating condition is determined in accordance with the base injection pulse width. The base fuel injection pulse is revised in response to the output of the O.sub.2 sensor so as to control the air-fuel ratio to the desirable value so that a final fuel injection pulse width corresponding to the amount of an actual fuel injection can be obtained. In this system, a temperature of the coolant of the engine is detected to compensate the fuel injection pulse width in accordance with the coolant temperature wherein the amount of fuel injection is increased under a warming-up condition. the increase of the fuel injection in the warming-up condition is continuously reduced in accordance with a gradual increase of the cooling water temperature from start-up to normal operation of the engine. It will therefore be understood that the air-fuel ratio is continuously changed from a rich mixture to lean mixture in accordance with the increase of the coolant temperature. According to this control, it is advantageous in the fact that there is no abrupt change in the air-fuel ratio so that a stable lean feedback control for the air-fuel ratio can be obtained to provide a proper drivability.
It should however be noted that there is a certain region of the air-fuel ratio which provides such engine operating condition that the amount of NO.sub.x in the exhaust gas is maximized causing an emission property of the engine to deteriorate, while the engine operating condition changes from the warming-up to normal operating condition. There occurs another problem in the system that the air-fuel feedback control produces a fluctuation in the air-fuel ratio in the vicinity of the desirable air-fuel ratio to harm the warming-up performance since the engine combustion is unstable under the warming-up condtition.